1. Field of the Invention
The invention relates to a radio altimeter with frequency modulation and more especially to a radio altimeter in which the transmitter and receiver share a common antenna.
2. Description of the Prior Art
Frequency modulated, continuous wave radio altimeters or FM/CW radio altimeters, are widely known.
The basic principles of continuous wave radars are set forth in the corresponding technical literature and more especially in the work by M. I. SKOLNIK entitled "Radar Handbook" chapter 16, edited in 1970 by McGraw-Hill. The main advantages of FM/CW radars reside in their relative simplicity of construction and in their ability to provide a precise measurement of the relative distance from a reflecting object situated inside their detection range. On the other hand, the FM/CW radars of the prior art have constructional restrictions and technical limitations which result from the need to use two separate antennae: a transmitting antenna and a reception antenna, generally, having identical electrical and mechanical characteristics. In fact, in order to provide sufficient radioelectric decoupling, the spacing between the two antennae must be large so as to eliminate the electromagnetic interferences between transmitter and receiver. There then arise certain radioelectric and mechanical problems due more especially to the fact that the antennae cannot be physically integrated in the equipment but must be fixed to the structure of the carrier vehicle and interconnected with the equipment through transmission lines whose transmission delay must be calibrated. In addition, these transmission lines introduce propagation losses and multiple reflections, which adversely affect the performance of the equipment. When the relative distance from the object to be detected is reduced, and when this object is of large size, a phenomenon of multiple paths may appear. The effect of this multipath phenomenon is to provide erroneous distance measurements. The geometry of the path of the signals radiated and picked up by the antennae is not perfect, and thus measurement errors may result when the altitude of the carrier is of the order of a meter. On the other hand, if we consider the mechanical problems raised by implantation of the two antennae on the structure of the carrier vehicle, the following should be mentioned: the need to reserve, inside the vehicle, housings for the antennae, passages for the radio frequency transmission lines and accesses to the connectors for mounting and removing the radio altimeter. Finally, the electric continuity of the wall of the carrier vehicle must be preserved, which involves limitations in the construction of this wall.
An FM/CW radio altimeter has already been proposed in which the transmitter and receiver operate continuously while using a common antenna. In this radio altimeter the local signal of the radio frequency mixer of the receiver is obtained by reflection of the signal transmitted by the T.O.S. (Standing Wave Rate) of the antenna. The construction of the equipment is thus appreciably simplified, but to the detriment of the minimum altitude measurable, which may reach 6 to 10 meters, depending on the maximum operating altitude of the radio altimeter.